metamorphic
Banded Gneiss
Gneiss (High-grade Regional Metamorphic Rock)
Hardness: 6-7 (Mohs scale); Color: Black, gray, tan, and yellow-brown bands; Luster: Dull to sub-vitreous; Crystal structure: Coarse-grained crystalline; Foliation: Distinct gneissic banding; Specific Gravity: 2.6 - 2.9
- Hardness
- 6-7 (Mohs scale)
- Color
- Black, gray, tan, and yellow-brown bands
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Black, gray, tan, and yellow-brown bands; Luster: Dull to sub-vitreous; Crystal structure: Coarse-grained crystalline; Foliation: Distinct gneissic banding; Specific Gravity: 2.6 - 2.9
Formation & geological history
Formed through high-grade regional metamorphism where pre-existing igneous or sedimentary rocks were subjected to intense heat and pressure, causing minerals to segregate into distinct layers. Found in the cores of mountain ranges and ancient continental shields.
Uses & applications
Used primarily in construction as crushed stone for road bases, as dimension stone for architectural facings, and as decorative landscaping stones. Highly polished specimens are used as countertops.
Geological facts
Gneiss is among the oldest known rocks on Earth; the Acasta Gneiss in Canada is dated to roughly 4 billion years old. The term 'gneiss' comes from a German word meaning 'sparkling' or 'to sparkle'.
Field identification & locations
Identify by looking for prominent 'gneissic banding'—alternating layers of light (felsic) and dark (mafic) minerals. Commonly found in riverbeds near mountainous regions or in glacial till. It will not fizz with acid (distinguishing it from marble).
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